1. Field of the Invention
The present invention is related to a system for filtering out unsolicited reply signals in Air Traffic Control (ATC) systems, and, more particularly, to a system for pre-filtering received target and fruit replies to filter out more obvious occurrences of fruit in the Air Traffic Control Radar Beacon System (ATCRBS).
2. Description of the Related Art
The Air Traffic Control Radar Beacon System (ATCRBS) has become an important source of surveillance information for Air Traffic Control (ATC), preempting "search radar". ATCRBS is an active system, whereby a ground based radar (interrogator) interrogates aircraft within its surveillance area for identity and altitude information Each sweep of the radar defines a receiving interval known as a Pulse Repetition Time (PRT). The aircraft responds by sending back a pulse train containing its assigned identity (Mode 3/A) or its flight level altitude (Mode C). These responses are known as beacon replies.
The ATCRBS is subject to a form of interference known as "fruit". Aircraft within range of the ATCRBS are not discretely addressed and there are often many interrogators within range of the aircraft. A reply received by the same interrogator that solicited the reply is called a target reply. A reply received by a first interrogator that was solicited by a second interrogator is defined as a fruit reply. Interrogators in close proximity to each other are operated asynchronously and often have differing Pulse Response Frequencies (PRFs) to help detect the fruit replies. The greater the number of aircraft and the number of interrogators, the more severe the fruit problem becomes. Fruit replies cause a form of interference with target replies known as garbling, and generally degrade the ability to detect and accurately locate ATCRBS equipped aircraft in an assigned surveillance area.
The advent of software oriented ATCRBS processing techniques has improved the beacon system capability in supplying surveillance data. With these techniques, the target replies are processed along with the fruit replies.
Fruit elimination processing (known as "defruiting") methods are known. Pulse-by-pulse defruiting involves filtering of individual video pulses of replies prior to decoding of the replies. Bracket defruiting involves filtering decoded replies. Both methods employ a comparison technique known as sequential observing, whereby a reply from an interrogation signal received in a first PRT interval (n) is compared to a reply from the interrogation signal received in a second PRT interval (n+j), where j represents the mode repeat interval. Since fruit is asynchronous, it will not usually repeat in the same time slot in subsequent PRT intervals on an interrogation-to-interrogation basis. A comparison of the replies received in adjacent intervals identifies the fruit replies.
These methods severely degrade the ATCRBS in areas in which moderate to high fruit rates occur. They are used, however, to lower the amount of data being processed by the software algorithms of the ATCRBS. Probability of detection of the fruit replies is lowered by miss multiplication (a true reply (n) followed by no reply (n+j) for a target signal). Azimuth accuracy is degraded by the loss of leading and trailing replies in azimuth as the antenna of the ATCRBS scans a given surveillance area.
Known beacon reply processing systems incorporate defruiting (using sophisticated algorithms) as an integral part of a sequential observer detection scheme. A sequential observer utilizes an "M of N" detector. The M of N detector has a pre-established threshold level (M) of received replies and a pre-established number (N) of PRT intervals over which the received reply signals are compared. For example, if the threshold level M is set to 6 and N=20 PRT intervals, then the M of N detector will count the number of replies received for a particular range position over 20 PRT intervals. If 6 or less are received, then the M of N detector indicates that the replies are fruit replies, and discards them. If more than 6 replies are received, the M of N detector sends them along to be processed by the ATC system. Fruit replies rarely occur M times within N PRT intervals, since they are asynchronous. A large quantity of fruit makes it difficult to economically provide the needed computing hardware to implement these sophisticated algorithms, since fruit elimination processing can consume nearly 50% of the provided processing capability needed to handle a typical surveillance area.
Surveillance areas having densities of 10,000 fruit replies per second can be adequately accommodated by modern computer technology. However, beyond this density value, the ability to efficiently and correctly process the surveillance area becomes limited. Fruit rates of up to 10,000 per second are typical at large airports (Los Angeles and New York) for terminal radars (60 nautical mile coverage) such as the Westinghouse radar system ASR-9. Fruit rates of up to 40,000 per second are typically encountered in the en-route series of radars (250 nautical mile coverage) both in the U.S. and Europe.
Ordinary computer systems are not fast enough to handle this high volume of processing. Large scale computing resources are prohibitively expensive for this application, even with multiprocessing capability, because of the large amounts of computing power needed to deal with high fruit rates.